The use in vehicles of the combination of an internal combustion engine and a conventional geared transmission is inherently inefficient. Because there is a single speed and load at which an internal combustion engine is most efficient, variations in load and the finite number of gear steps in conventional transmissions result in operation away from the highest efficiency region. Moreover, the need for sudden increases in power requires high powered engines, which are usually under utilized.
Continuously variable transmissions--which are capable of producing a continuously variable output for a constant input are more efficient than geared transmissions, because they enable an internal combustion engine to be operated at a constant, most efficient speed. Continuously variable transmissions of various types have been known for many years. For example, U.S. Pat. No. 197,472, granted to Hunt on 27 Nov., 1877, discloses a continuously variable transmission of the roller-friction type, comprising a first toroidal disc mounted on the end of a drive shaft, a second opposed toroidal disc mounted on an output shaft, and an adjustable roller mounted on a fixed base between the two discs. The roller contacts the opposing faces of the two discs, with the result that as the first disc is rotated in one direction by the drive shaft, the second disc is rotated in the opposite direction. The roller can be tilted so as to contact the two discs at different radii from the axis of rotation of the discs. Since the tangential velocity at a given point on a disc is equal to the angular velocity of the disc multiplied times the radius (the distance between such point and the disc axis of rotation), and since the tangential velocity of the roller at the point of contact with the driven disc is necessarily equal to the tangential velocity of the roller at the point of contact with the driven disc, tilting the roller to contact the discs at unequal radii causes the output disc and the output shaft to be rotated at various angular velocities both slower and faster than the drive shaft velocity. The Hunt transmission is a friction drive, and is restricted to relatively low power and/or intermittent use application, but over the years, developments in tractive fluids and thrust bearing technologies have increased interest in the use of this general type of continuously variable transmissions in vehicles such as the automobile.
However, these continuously variable transmissions have certain inherent limitations, including:
(1) It is not possible to obtain a zero output velocity, so recourse must be had to clutches or other means for disengaging the transmission from the input or the output shaft, for purposes of idling or stopping.
(2) The speed of the output shaft cannot be continuously varied from forward through zero to reverse, so externally shifting gearing must be provided to reverse the direction of the vehicle.